1. Field of the Invention
The present invention relates to a liquid crystal display device and a lightguide.
2. Description of the Related Art
A liquid crystal display device is widely used as a display device for a television set or an information communication terminal such as a computer. The liquid crystal display device controls the image to be displayed by changing the orientation of liquid crystal molecules to change the light transmittance. Liquid crystal itself does not emit light, and hence a reflective liquid crystal display device uses sunlight or light in the room by reflecting the light, or the liquid crystal display device uses an illuminating device called a backlight unit. As a structure of the backlight unit, a direct type backlight unit, which includes a light source on a rear side of a display surface of a liquid crystal panel, and a side-light type backlight unit, in which light emitted from the side is reflected to illuminate a display surface of a liquid crystal panel, are mainly known. In recent years, a light emitting diode (LED) has been increasingly used as the light source of the backlight unit. Japanese Patent Application Laid-open No. 2002-184225 discloses a backlight unit using the LED.
In the side-light type backlight unit using the LED, a member called a lightguide (light guide plate) is used. The lightguide reflects light entering from the side of the display surface of the liquid crystal panel toward the display surface. FIG. 13 is a view illustrating an example of a conventional lightguide 200. The lightguide 200 includes a light exit surface 220 which causes the light to exit toward the liquid crystal panel. The lightguide has a rectangular shape when viewed from the direction of the light exit surface 220. The lightguide includes a plurality of LED light sources 60, which are provided in contact with a light entrance surface 210, which is a side surface corresponding to one side of the opposing short sides of the rectangular shape. The plurality of LED light sources 60 are arranged in a direction in which the one side of the lightguide extends. A reflection surface of the lightguide is opposed to the light exit surface 220, and the reflection surface reflects part of light emitted from the LED light sources 60 and traveling inside the lightguide 200 toward a direction close to the outward normal of the light exit surface 220. The reflection surface has a structure capable of reducing unevenness of the light exiting from the light exit surface 220, in particular, unevenness of the light in the long side direction of the rectangular shape when viewed from the direction of the light exit surface 220.
On the light exit surface 220, a plurality of line-shaped protrusions 221 are formed, which extend from a position spaced apart by a fixed clearance from one side on the light entrance surface 210 side, toward the opposite side, which is on a side of a surface opposed to the light entrance surface 210. The line-shaped protrusion 221 has a cross section in, for example, an arc shape or a V-shape. When the light is reflected at the position of the line-shaped protrusion 221, the line-shaped protrusion 221 weakens the light component in a direction orthogonal to the extending direction of the line-shaped protrusion 221, to thereby make the light beams exiting from the light exit surface 220 the same direction.
When the side-light type backlight unit using a point light source such as the LED is used, there arises the following problem. That is, there occurs a phenomenon that, in a region of a display region of the liquid crystal display device, which corresponds to the vicinity of the light entrance surface, a part close to any of the point light sources is bright, and a part far from all of the point light sources is dark (so-called luminance unevenness). Therefore, for example, it has been necessary to separate the light entrance surface and the display region by an approximately fixed clearance.